ACTION OF PLANTS ON THE SOIL. 259 



the ground. If one were to mark out the space of ground from which the lime, 

 potash, and other nutrient salts used in the construction of a birch-tree were 

 derived, its bulk would certainly be found to be much larger than that of the birch; 

 and, if we were to try to estimate the volume of air through which the carbon, 

 which has been converted into organic compounds in the tree, was previously dis- 

 tributed in the form of carbon dioxide, it would turn out to exceed the volume of 

 the birch a thousandfold. In this sense, every plant may justly be considered as 

 an accumulator of those substances which serve for its nutriment. Every plant 

 continues, so long as it lives, to store them up in ever-increasing quantities in its 

 own body, and in the case of long-lived plants there is thus collected ultimately 

 quite a considerable quantity. When the life of an accumulator of the kind is 

 extinguished, those materials which were taken from the atmosphere are able to 

 return into the atmosphere; but such mineral food as has been derived from the 

 ground and lifted into the upper parts of the plant — particularly those above the 

 ground — and has there been amassed in a confined space, does not return to its 

 original place. A dead tree breaks down on the first provocation, and the trunk 

 lies on the ground and rots. Such part of its substance as can pass into the atmos- 

 phere in gaseous form escapes; but the salts accumulated within it, which it raised 

 from deep under ground during its lifetime, are retained by the surface-layers of 

 the soil. Even though some of them are washed out of the trunk by the lixiviating 

 action of rain-water, the superficial layers of earth operate as a filter, and do not 

 allow any part to return to the underlying strata. So, too, the nutrient salts which 

 reach the foliage of plants are added to the top layers of the soil; for fallen leaves 

 go through much the same process as the trunk which is broken by storms and 

 undergoes decay as it lies prostrate upon the ground. 



Thus, wherever men do not interfere by clearing away the accumulative agents 

 in question, i.e. plants; where there is no removal of the haulms of cereals from 

 fields, or of mown grass and herbs from meadows to serve as hay, or of timber from 

 the forest — wherever, in a word, the vegetable world is left to itself and the 

 natural progress of evolution is not frustrated by any disturbing element — -the 

 food- salts which have been amassed will accumulate in the uppermost layers of the 

 earth. Moreover, seeing that, as has been already pointed out, every plant has the 

 power of possessing itself of substances of value to it, even when they are only 

 present in the environment of the roots in scarcely appreciable quantities, it is 

 possible for the top layers of soil to contain a considerable amount of a substance 

 which only occurs in the subjacent rock in such small measure as to be detected 

 with difficulty. The percentage of lime yielded by the subsoil on the Blockenstein, 

 a granitic mountain 1383 meters high, on the borders of Bavaria and Upper Austria, 

 was 2-7, whilst that of the top layer was 197; the percentage on Mount Lusen, 

 situated to the north of the Blockenstein, was 19 for the subsoil and 8'6 for the 

 superficial layer. When one considers that fresh plants strike root in the ground 

 near the surface and these again act as accumulators, and remembers in addition 

 that snails make their appearance in abundance wherever vegetable food containing 



